A man and woman found unconscious in Wiltshire were poisoned by Novichok - the same nerve agent that poisoned ex-Russian spy Sergei Skripal, police say.
The couple, believed to be Charlie Rowley, 45, and Dawn Sturgess, 44, fell ill at a house in Amesbury on Saturday and remain in a critical condition.
Police say no one else has presented with the same symptoms.
There was "nothing in their background" to suggest the pair were targeted, the Met Police said.
Metropolitan Police Assistant Commissioner Neil Basu said it could not be confirmed whether the nerve agent came from the same batch that Mr Skripal, and his daughter Yulia, were exposed to.
But he said the possibility was "clearly a line of enquiry".
Mr Basu said no contaminated items had yet been found, but officers were putting together a "very detailed examination of [the couple's] movements" in order to determine where they were poisoned.
He added that members of the public should not pick anything up if they don't know what it is.
"We have no idea what may have contained the nerve agent at this time," he said.
The Counter Terrorism Policing Network is now leading the investigation, working with Wiltshire Police.
The BBC's security correspondent Gordon Corera said: "The most likely hypothesis is that this is leftover Novichok from the attack on the Skripals back in March."
Chemical weapons expert Richard Guthrie said it was possible that the Novichok which poisoned the Skripals may have been disposed of "in a haphazard way".
If the couple had come across it in a syringe or pot, it might have been better preserved, he told BBC Breakfast.
England's chief medical officer, Sally Davies, said: "I want to reassure the public that the risk to the general public remains low."
The Skripal episode meant officials had a "well-established response" in place, she said.
"As before, my advice is to wash your clothes and wipe down any personal items, shoes and bags, with cleansing or baby wipes before disposing of them in the usual way.
"You do not need to seek advice from a health professional unless you are experiencing symptoms, as any individual who had been significantly exposed at the same time would by now have symptoms."
- What are Novichok nerve agents?
- Russian spy: What happened to the Skripals?
- Medics who saved Skripals reveal fears
On Saturday, paramedics were called twice to the property in Amesbury - in the morning, after Ms Sturgess had collapsed, then later the same day, after Mr Rowley had also fallen unwell.
"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs," Wiltshire Police said.
The news that Novichok was to blame was announced following analysis at the defence research facility at Porton Down, Wiltshire.
As a precautionary measure, sites in Amesbury and Salisbury, believed to have been visited by the couple before they fell ill, have been cordoned off.
There is no evidence to suggest either visited the sites that were decontaminated following the poisoning of Sergei and Yulia Skripal.
Local residents have been warned to expect to see an increased police presence - including officers wearing protective equipment.
Security correspondent Gordon Corera said the poisoning was "hugely significant" as the public "will be worried about public health".
He added: "Perhaps this is some of the Novichok prepared for the attack [in Salisbury in March] and discarded - maybe somewhere like a park, a house - and maybe these two came across it."
He added it could give counter-terrorism investigators new leads on where the nerve agent was "brought and put together" before the attack on the Skripals.
Home Secretary Sajid Javid said his thoughts were with the two individuals affected and thanked the emergency services and staff at Salisbury District Hospital.
He said the events follow "the reckless and barbaric attack which took place in Salisbury in March".
"The government's first priority is for the safety of the residents in the local area but as Public Health England has made clear, the risk to the general public is low," he said.
"Tomorrow [Thursday] I will chair a meeting of the government's emergency committee Cobra in relation to the ongoing investigation."
bbc.com
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